1. Field of the Invention
This invention relates to an acellular red blood cell substitute comprising an essentially tetramer-free, cross-linked, polymerized, pyridoxylated hemoglobin solution which is free of stromal contaminants. It further relates to a method of preparing the acellular red blood cell substitute.
2. Description of the Related Art
For several years, stroma-free hemoglobin has been known in the art to have oxygen transport and reversible oxygen (or ligand) binding capacities. Since toxicity problems have precluded use as a blood substitute, stroma-free hemoglobin has required further modifications to provide a nontoxic, useful pharmaceutical product. In U.S. Pat. Nos. 4,001,200; 4,001,401 and 4,053,590, a polymerized, cross-linked, stroma-free hemoglobin is disclosed as a blood substitute for carrying oxygen to tissues and organs and as a blood plasma expander. Pyridoxylating the stroma-free hemoglobin has been shown to favorably alter reversible oxygen binding capacities and increase the stability and shelf-life of the biological product. J. Surgical Research 30: 14-20 (1981). Pyridoxylated hemogobin polymerized with glutaraldehyde has been described and characterized in L. R. Sehgal, et al., In Vitro and In Vivo Characteristics of Polymerized, Pyridoxylated Hemoglobin Solution, Fed. Proc. 39: 2382 (1980); L. R. Sehgal, et al., Preparation and In Vitro Characteristics of Polymerized, Pyridoxylated Hemoglobin, Transfusion 23 (2): 158 (March-April 1983). Further, the ability of the polymerized, pyridoxylated hemoglobin to act as an oxygen carrier has been disclosed in L. R. Sehgal, et al., Polymerized, Pyridoxylated Hemoglobin: A Red Cell Substitute with Normal Oxygen Capacity, Surgery 95 (4): 433-38 (April 1984); L. R. Sehgal, et al., An Appraisal of Polymerized, Pyridoxylated Hemoglobin as an Acellular Oxygen Carrier, Advances in Blood Substitute Research 19-23 (Alan R. Liss, Inc. 1983).
For years investigators have reported that hemoglobin solutions prepared by various techniques, while capable of carrying sufficient quantities of oxygen to support life, have undesirable side effects. The most troubling side effect is a decrease in kidney performance. These changes were thought to be due to the presence of unwanted contaminants such as bacterial endotoxin or fragments of red cell membranes (stroma). While contaminants such as these can indeed produce renal alternations, hemoglobin solutions essentially free of the above contaminants still produce substantial renal dysfunction. Although this dysfunction is temporary and reversible, it can be very alarming in a clinical situation such as hemorrhagic shock, as the kidney is already at risk in this low blood flow state. The cause for the renal dysfunction has been ascribed to physiologically unacceptable amounts of unpolymerized hemoglobin tetramer. Other undesirable side effects of the infusion of tetrameric hemoglobin are renal toxicity, vasoconstriction, hemoglobinurea, depression of heart rate, elevation of mean arterial blood pressure and extravasation of infusate especially into the peritoneal cavity.
In practice, no known hemoglobin-derived blood substitute has been successful in totally avoiding toxicity problems. These products prepared according to the state of the art have been found to contain varying amounts of hemoglobin tetramer. For example, the process of preparation according to U.S. Pat. Nos. 4,001,200; 4,001,401 and 4,053,590 does not provide a therapeutically useful product. First, like other processes, there is an undesirable, high amount of unpolymerized hemoglobin tetramer in the final product. Secondly, too many contaminants such as toxic residual toluene may remain in the solution since they may not be completely removed during preparation. Thirdly, the product, described as having a P.sub.50 of 100-120 mm Hg, would be nonfunctional physiologically in that the hemoglobin solution would not pick up oxygen in the lungs. Lastly, increased proportions of higher molecular weight polymers yield a product of high gelation lability such that subsequent steps of filtration and purification are difficult or impossible to accomplish except in unacceptably dilute solutions.